bod

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BOD
Water bodies both produce and consume oxygen. It gains oxygen from the atmosphere and from plants as a result of photosynthesis. For flowing water it would dissolve more oxygen because of the effect of its churning and continuous movement. Waste water containing organic materials are decomposed by microorganisms that utilize the oxygen within the stream. The Biochemical Oxygen Demand (BOD) is the amount of oxygen consumed in this process. Other sources of oxygen consuming waste include storm water runoff from farm areas or septic systems. In this aspect, oxygen is measured in its dissolved form (DO). If more oxygen is consumed than produced, it would affect the eco-system sustained by the waterway or transfer its oxygen consuming inhabitants to lower profile water bodies.
According to APHA, 1992 in their study “Standard methods for the examination of water and wastewater” DO levels fluctuate throughout the day and are influenced by temperature and altitude. The study also states that cold water has a higher oxygen holding capacity than warmer water. Water at 0C had a DO level of 14.60mg/L compared to water at a temperature 36C at a value 6.82mg/L. Similarly, in March 1997 at The University of Rhode Island in the Department of Natural Resources highlighted that salinity, altitude, groundwater inflow and water temperature are some of the factors that influence dissolved oxygen in a water body. The higher the salinity in water reduces the amount of oxygen dissolved. Warm water normally holds less oxygen in solution than cold water. Thermal stratification (heating across different layers) causes depletion of DO and eventually completely depletes levels because there is no replenishment. According to an article by Simpson (1991), ...

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... dichromate, under acidic conditions. A known excess amount of this oxidant is added to the sample and once the process is completed, the COD concentration value is calculated by measuring the remaining amount of oxidant in the solution. This process is usually done through titration by use of an indicator solution. It is measured in milligrams per liter, and indicates the mass of oxygen consumed per liter of solution.
While the Biochemical Oxygen Demand (BOD) test requires 5 days, the COD test only takes about 2-3 hours for full completion and analysis. Within this test it measures all the organic contaminants present in the wastewater sample, including those that are not biodegradable. For the BOD test, toxic compounds such as heavy metals cannot be analyzed therefore, the COD test can be used to measure the strength of wastes that are too toxic for the BOD test.

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